1. Field of the Invention
This invention relates to a method of deploying a fixed wireless access communications network such that a specified level of link performance is maintained. The invention also relates to a computer system for implementing the method, a computer program for controlling the computer system and a communications network deployed using the method.
2. Description of the Prior Art
Fixed wireless access communications networks use a fixed antenna at each subscriber premises where the antenna is typically directional. Each subscriber antenna communicates with a base station (to which it is directed, in the case that direct antennas are used). Each base ton is in turn connected to a physical communications network such as a public switched telephone network via a transmission link. A typical base station supports many subscribers, for example 500 to 2000 subscribers in the IONICA (trade mark) system. The resulting fixed wireless access system is capable of delivering a wide range of access services such as POTS (public operator telephone service), ISDN and broadband data.
When a fixed wireless access communications system is initially deployed, a base station of a particular capacity is installed to cover a particular populated area. The capabilities of the base station are designed to be commensurate with the anticipated coverage and capacity requirement. Subscribers' antennas are mounted on a building, for instance, on a chimney, and upon installation are normally directed towards the nearest (or best signal strength) base station or repeater antenna (any future reference to a base station shall be taken to include a repeater).
In order to meet the capacity demand, within an available frequency band allocation, fixed wireless access systems divide a geographic area to be covered into cells. Within each cell is a base station with which the subscribers stations communicate; the distance between the cells refusing the same frequency being determined such that co-channel interference is maintained at a tolerable level. When the antenna on the subscriber premises is installed, an optimal direction for the antenna is identified using monitoring equipment. The antenna is then mounted so that it is positioned towards the optimal direction.
As already mentioned, fixed wireless access systems divide a geographic area to be covered into cells. For initial planning and design purposes these cells may be represented as hexagons, each cell being served by a base station (in the centre of the hexagon) with which a play of subscriber stations within the cell (hexagon) communicate. When detailed cell planning is performed the ideal hexagonal arrangement can start to break down due to site constraints or for radio propagation reasons. The number of subscriber stations which can be supported within each cell is limited by the available number of carrier frequencies and the number of channels per frequency.
Base stations are expensive, and require extensive effort in obtaining planning permission for their erection. In some areas, suitable base station sites may not be available. One problem in fixed wireless access system design is to have as few base stations as possible, whilst supporting as many subscriber stations as possible. This helps to reduce the cost per subscriber in a fixed wireless access system. An on-going problem is to increase the traffic carrying capacity of base stations whilst at the same time keeping interference levels within acceptable bounds. This is referred to as trying to optimise or increase the carrier to interference level ratio. By increasing the traffic capacity the number of lost or blocked calls is reduced and call quality can be improved. (A lost call is a call attempt that falls.)
Cells are typically grouped in clusters as shown in FIG. 1. In this example, a duster of seven cells is shown. Within each cluster 7×6=42 frequencies am each used once. The term “frequency re-use factor” is used to refer to the number of sets of frequencies that the total number of available frequencies is divided into. In this case, the frequencies are divided into 7 sets, one for each cell in a cluster.
FIG. 2 shows how a larger geographical area can be covered by re-using frequencies. In FIG. 2 each frequency is used twice, once in each cluster. The frequency re-use factor N is 7: Co-channel interference could occur between cells using the same frequencies and needs to be guarded against through cell planning. When the capacity of a cell or duster is exhausted one possibility is to sectorize each cell. This involves using directional antennas on the base station rather than omnidirectional antennas. The 360° range around the base station is divided up into a number of sectors and bearers are allocated to each sector. In this way more bearers can be added whilst keeping interference down by only using certain frequencies in certain directions or sects. The frequency reuse factor is a product of the base re-use factor and the sector re-use factor.
Known approaches for seeking to increase system capacity include fixed frequency planning (FFP) which involves carefully planning re-use patterns and creating sector designs in order to reduce the likelihood of interference. For example, FIG. 3 shows an example of a fixed frequency plan with a frequency re-use factor N of 4 and which is known as the “mirror method”. Each sector with the same reference numeral is constrained to use a specific set of frequencies that are different from the frequencies used by sectors with different reference numerals. There are four different reference numerals 31, 32, 33, 34, for each of four different frequency sets. The letters H and V are used to denote horizontal and vertically polarised frequency channels. However, fixed frequency planning is problematic because it is often difficult to map a frequency plan onto an actual communications network. This mapping process is complex time consuming and adds to cost.
As well as this fixed frequency planning is limited in that it is difficult to maximise capacity whilst at the same time maintaining specified link performance levels for all subscribers. Whilst the fixed frequency plan aims to reduce interference, specified link performance levels are not maintained for all individual subscribers. As subscribers are added to the fixed frequency planned network, effects on the link performance provided to other subscribers result. Fixed frequency planned communications networks thus typically give rise to some subscriber locations for which link performances are relatively low compared with other subscriber locations in the same communications network. In order to ensure no problem links in a fixed frequency planning network, then a very large frequency re-use factor would be required. However, this is not practical because the high re-use factor would lead to a low capacity.
Thus it is difficult to provide a method of frequency planning for a fixed wireless access communications system such that specified link performance levels are maintained and capacity is maximised whilst at the same time the problems associated with fixed frequency planning are reduced. The term “fixed frequency plan” is used to refer to a scheme which specifies a regular pattern of base station locations and a pattern of frequency use for communications links between those base stations and subscriber stations. When a communications network is deployed according to a fixed frequency plan, the base station locations and pattern of frequencies used for the communications links needs to adhere to the fixed frequency plan closely, in order for the benefits of the fixed frequency plan to be achieved.
It is accordingly an object of the present invention to provide a method of deploying a fixed wireless access communications network such that a specified level of link performance is maintained which overcomes or at least mitigates one or more of the problems noted above.